x68 
Nature's Scavengers. 
[April, 
which are powerful purifiers. Like all the higher forms of 
vegetable life they decompose carbonic acid under the influ- 
ence of light, and evolve oxygen, by which the impurities 
are literally burnt up; Among such plants the common 
duckweed holds no undistinguished place. 
Animal scavengers exist, also, which eat up the putrid or 
putrescent matter. Some work of this nature is done by 
aquatic birds — by none more eagerly than by the common 
duck. Any dead body floating in the water, whether that of 
a mammal, bird, or fish, is greedily gobbled up by these un- 
clean feeders. We doubt whether any reptile or amphibian 
can be called a purifier of the water. The frog is commonly 
supposed to exercise this function, and its presence in wells 
is therefore viewed with approval. Now there is no doubt 
that it consumes the larvae and ova of inserts, — matter by 
no means desirable to be introduced into the human stomach, 
- — and thus to some extent improves the water ; but it seeks 
living prey, and we have never known it consume any dead 
or putrid matter, whether of animal or vegetable origin. 
Fishes— for example, the eel — and Crustaceans are diligent 
consumers of dead bodies floating in or sunk under water. 
The shrimp, by its performances of this kind, has earned 
the title of the “ scavenger of the ocean.” But we have no 
reason to believe that either fish or Crustacea can deal with 
those minute, almost pulpy, fragments of decomposing 
matter which form so large a proportion of the pollutions of 
lakes, rivers, and even seas. Here, as far at least as fresh 
water is concerned, the task is taken up by inserts. It is 
well known that many Diptera and Neuroptera, though 
winged creatures, when arrived at maturity, begin their life 
in the waters. Many of these, when larvae, feed upon this 
very pulp of decomposing animal and vegetable matter to 
which we have just referred. To this class belong the gnats 
and mosquitoes. It is accordingly maintained that the an- 
noyance which they occasion when mature is compensated, 
and even outweighed, by their sanitary services when larvae. 
Without their aid, it is said, certain districts in tropical 
climates would be absolutely uninhabitable on account of 
malaria. We must confess to no small amount of scepticism 
on this subject . We find, first, mosquitoes very prevalent 
where malaria is malignant. Here, then, is a proof that 
the mosquitoes, if they contribute anything to the health of 
a district, fall incalculably short of what is requisite. Again, 
we find them swarming in countries where malaria is un- 
known, e.g., in Lapland, and where, from the nature of the 
climate, it seems not probable to arise. There are, also, 
